The exhaust system of an engine (internal combustion engine) is usually equipped with a catalyst, such as a three-way catalyst or a NOx storage catalyst, for purification of exhaust. Downstream of the catalyst, an oxygen concentration sensor may be provided for exercising air-fuel ratio control.
A technology for switching the fuel injection mode of the internal combustion engine according to load or the like is also known. The fuel injection mode includes a normal operation mode in which an operation is performed in a stoichiometric or lean state, for example, with the air-fuel ratio of the exhaust being provided as a feedback; an enriched operation mode in which the air-fuel ratio is subjected to open loop control for enrichment in the case of a high load or the like; and a fuel cut mode in which fuel injection is stopped under no-load conditions such as during deceleration.
A proposal has been made for a method which, in failure determination for detecting the deterioration of an oxygen concentration sensor with the use of enrichment after release of the fuel cut mode, prohibits monitoring if an exhaust gas flow rate integrated value is less than a predetermined amount (see Japanese Patent No. 3676641).
A proposal has also been made for a method which comprises detecting a failure from an output of an oxygen sensor when controlling a target air-fuel ratio during travel from a lean value to a rich value (see Japanese Patent No. 4548443).